Laser/infrared tracking systems exist for both air and land-based applications. One application is a counter measure to man-portable air-defense systems (MANPADS), small anti aircraft missiles. These countermeasure systems operate with tight tolerances that are difficult to achieve. For example, bore sight alignment is a demanding issue. Typically, bore sight measurements require accuracy of the order of 10 μrad. This is difficult to measure without interference from artifacts. It is also difficult to maintain, so these measurements should be made frequently.
Retro reflection is also a particularly important issue. The jam laser is typically very powerful and the tracker very sensitive. If the tracker sees even a small fraction of the retro reflected jam laser from the measurement apparatus, it may begin to track that instead of the reference target, a phenomenon called “self tracking.” Retro reflection from most devices and surfaces used in beam profile and measurement cameras is typically several orders of magnitude larger than the limits for the onset of this effect for most trackers. Practical experience has shown that elimination of this artifact is difficult both to achieve and to verify.
Measurement of the pointing of a beam directed under control of a tracker requires an instrument that both locates the beam in angular space and isolates it from the tracker providing the pointing, unless closed loop tracking (around the backscattered beam) is the intent.
Test devices have generally been ad-hoc devices built without an understanding of back scatter and how to suppress it. Backscatter contaminates pointing device management and can require unrealistic input signals for tracking (i.e. increasing it enough to overwhelm the backscatter component). These Ad hoc methods are unreliable and involve high maintenance.
FIG. 1 shows a prior art configuration 100 of the standard method to provide bore sight alignment between the outgoing target beam and the incoming directed beam by replacing unit under test (UUT) 105 with corner cube 110. Corner cubes of appropriate size and precision are commercially available. The problem is that beam camera 115 is so insensitive (because it must survive and measure a high power laser) that it is difficult to get sufficient beam power from a black body source 120. The source 120 is usually operated at much higher power levels than required for tracking by UUT 105.
The alternative, often used, is to add a reference laser beam to augment the black body tracker output. While this is useful it too is susceptible to misalignment, adding to the overall complexity. With a compact pointing instrument, the mount is smaller and more rigid making reference lasers less prone to lose alignment themselves. Typically, a reference laser is used to get near alignment and use the black body for a final measurement, either to achieve final alignment or to measure the residual error.
FIG. 2 further depicts a prior art configuration 200 for the problem of testing and pointing a directable beam pointing device. The unit under test is jammer 205 comprising tracker 210 and jam beam source 215. Tracker 210 uses a target beam generated at source 220 and collimated by lens 225 to point jam head 205. The camera comprising lens 230 and focal plane array package 235 measures the centroid and profile of the beam. The objective is to measure the departure from anti parallel of arrows 240 and 245. The problems in making this measurement are: 1) Adjusting and maintaining calibration of the alignment of the camera 235 and target beam and 2) Retro reflections such as indicated by arrow 250. FIG. 2 shows the retro reflection path as the dashed arrow 250.
Laser retro reflection is assumed to be a spherical wave front leaving the measurement camera. When it returns to the lens used by the beam analysis camera, it is collimated and sent back towards the UUT where it can enter its tracker aperture and be imaged in competition with the black body produced target.
What is needed is a test method and device that is simple to build, rugged, reliable and has backscatter reliably reduced to prevent self tracking. Such a device can be used for lab testing, alignment check prior to range testing, flight line testing of installed jammers, and calibration of all multi aperture directable jammers.